1. Field of the Invention
The present invention relates generally to composite laminated sandwich panels comprised of the following layers (i) aluminum—(ii) glass fibre with adhesive—(iii) aluminum. These panels include those known under the generic name “Glare.” The present invention is further directed to the use of such composite laminated sandwich panels in aeronautical construction, and particularly as a fuselage skin.
2. Description of Related Art
Laminated aluminum—organic polymer-based fibre—aluminum composites have been known for a long time. The organic polymers are typically polyamides or polyesters. For example, EP 0 056 288 B1 and EP 0 056 289 B1 (Technische Hogeschool Delft) disclose a laminated material including two thin sheets of heat treated aluminum (particularly alloy 2024 T3 and alloy 7075 T6) surrounding an adhesive layer, in which polyparaphenylene—terephthalamide (PPDT) fibres with a high modulus of elasticity (between 50 GPa and 250 GPa) are embedded. Several alternating layers of organic material and aluminum may be superposed in this way. This type of organic fibre-based material cannot be used at high temperatures, typically above 120° C. to 130° C., for prolonged periods. Furthermore, it is difficult to control residual stresses in these composite laminated sandwich panels.
EP 0 312 150 B1 (Structural Laminates Company) discloses the use of carbon, aramide, polyethylene or glass fibres in the form of continuous filaments running parallel to each other in at least one direction. These fibres are impregnated with a thermoplastic adhesive. The composite laminated sandwich panels or panels are prestressed.
EP 0 323 60 A1 (Akzo Nobel) discloses a process for manufacturing structural elements for an aircraft fuselage including composite laminated sandwich panels using thermosetting resins, and particularly epoxy resins. Those skilled in the art would denote these composite laminated sandwich panels by the generic name “Glare”.
U.S. Pat. No. 5,547,735 (Structural Laminates Company) describes the use of sheets made of 5052 alloy in composite laminated complexes. U.S. Pat. No. 4,657,717 (Alcan International) describes the use of superplastic sheets in a laminated complex. WO 98/53989 (Akzo Nobel) discloses a process for making composite laminated sandwich panels including sheets made of aluminum, copper, magnesium, steel or titanium-base alloys. The preferred types of alloys are 2×24 T3 and 7×75 T6 type alloys.
The state of the art, manufacturing processes, characteristics and applications of Glare type laminated aluminum—glass fibre complexes are summarised in the article “Fibre Metal Laminates for High Capacity Aircraft”, by A. Vlot, L. B. Vogelesang and T. J. de Vries, 30th International SAMPE Technical Conference, Oct. 20–24, 1998, pages 456–470, as well as in the article “The Residual Strength of Fibre Metal Laminates: Glare 2 and Glare 3”, by C. A. J. R. Vermeeren, 30th International SAMPE Technical Conference, Oct. 20–24, 1998, pages 471–482, both of which are incorporated herein by reference in their entireties.
The term “heat treatable alloy” is defined for aluminum in standard EN 12258-1 as being an “alloy that can be hardened by an appropriate heat treatment”. The opposite term “non-heat treatable alloy” is defined in the same standard as being an “alloy that cannot be substantially hardened by heat treatment”. Heat treatable alloys include alloys in the 2xxx, 7xxx and 6xxx series, while alloys in the 5xxx and 3xxx series are non-heat treatable alloys.
Typically, composite laminated sandwich panels according to the state of the art comprise N thin sheets made of heat treatable aluminum alloy alternating with N-1 sheets of epoxy resin reinforced with glass fibres. The reinforced epoxy resin sandwich panels are sometimes called “prepregs”. In such assemblies, the aluminum sheets form the outside faces of the composite laminated and alternate with glass fibre reinforced resin sheets. Heat treatable alloys used are alloys typically belonging to the 2xxx or 7xxx families, and more particularly the 2024 alloy in the T3 state. The thickness of the said thin aluminum alloy sheets is typically of the order of 0.1 mm to 0.6 mm. The outside sheet of the laminated complex is normally a clad plate. A clad plate is utilized in order to minimize corrosion of the outside face. The total thickness of the laminated complex depends on the application. The thickness can be on the order of about 3 mm for a fuselage skin, while a thickness on the order of about 20 mm may be necessary for a skin stiffener for an aircraft door (see article by B. Isink “Mit Glare ‘erleichtert’ abheben”, published in the Airbus News Review, Nov. 1, 2001, incorporated herein by reference in its entirety).
There are some disadvantages with these composite laminated sandwich panels according to the state of the art. First, thin heat treatable aluminum sheets are expensive because their manufacturing process is complex. Furthermore, most alloys in the 2xxx and 7xxxx families, and particularly those alloys in the 2xxx and 7xxx families that are used for making structural elements for aeronautical applications, are sensitive to corrosion. This is why clad sheets are used as the outside face of the structural element when this sensitivity to corrosion is a problem.
In general, conducting a heat treatment on a thin sheet is difficult, since a thin sheet can deform, particularly during quenching. Thus, if a heat treatment is to be conducted, corrective measures are made that require additional steps in the process, for example smoothing, flattening or controlled tension.
Conducting a heat treatment of thin clad sheets is even more difficult, since some chemical elements in the core can diffuse into the cladding at high temperature. For example, if the copper content in a 2xxx alloy such as 2024 enters the cladding layer, this layer will no longer satisfactorily play its intended corrosion protection role and can be attacked by the environment. Furthermore, since a cladding layer is mechanically weak, thin clad sheets must be handled with care to prevent surface scratches that could become corrosion sites. Consequently, thin clad sheets are significantly more expensive to make, and moreover produce a higher scrap rate during their manufacture than with non-clad sheets made of the same alloy and with the same thickness and metallurgical temper. This further increases their cost to manufacture and ultimately their final sales price to customers. The problem of being susceptible to scratching continues even after a Glare type laminated complex is manufactured. That is, if an accidental scratch causes an unacceptable defect during manipulation of such a complex, the part has to be scrapped. As such the entire costs associated with the manufacture of the scrapped laminated complex is lost.
Therefore, it would be desirable to have composite laminated aluminum—glass fibre sandwich panels that use thin aluminum alloy sheets that can be made using a simpler and more reliable and cost effective manufacturing process than the traditional processes described above that have been previously utilized with thin sheets made from the 2xxx and 7xxx alloys. More particularly, it would be desirable for the thin plates to be less sensitive to corrosion than thin plates made with 2xxx or 7xxx type alloys, and for composite laminated sandwich panels made using these plates to have mechanical characteristics comparable to the characteristics of known composite laminated sandwich panels.